1. Field of the Invention
This invention relates to the electronic detection of article theft and more particularly it concerns improvements in the detection of special electronic circuits, known as "targets", which are carried on protected articles.
2. Description of the Prior Art
Electronic article theft detection systems of the type to which this invention applies incorporate a monitor set up at an interrogation zone, such as the exit from a store, library or other area in which protected articles are kept. The protected articles are provided with special targets capable of producing a predetermined electromagnetic field disturbance when they are taken through the interrogation zone and this disturbance is detected by the monitor which in turn actuates an alarm. Authorized passage of the protected article is made possible by removal or deactivation of the target with a special tool or by allowing the article to be taken through a special bypass passageway.
One prior art electronic theft detection system that has been especially successful is shown and described in U.S. Pat. No. 3,500,373. As described in that patent, the monitor includes an antenna which generates in the interrogation zone an interrogating electromagnetic field whose frequency varies cyclically or sweeps at a predetermined rate over a predetermined frequency range. The targets, which are fastened on the protected articles, comprise resonant electrical circuits which resonate at a frequency within the predetermined frequency range. As the frequency of the interrogating field sweeps back and forth across the resonant frequency of a target being carried through the interrogation zone, a series of disturbances, in the form of pulses, is generated. These disturbances are sensed by means of an antenna forming part of the monitor. The antenna converts these disturbances to electrical signals which are detected and used to activate an alarm.
One characteristic common to most electronic theft detection systems is that the signal level or amplitude of the electromagnetic field disturbance produced by the target is extremely low. This is due to several factors. Firstly, in most instances, the target is passive and generates no electromagnetic energy of its own. Secondly, the target must be very small so that it can be affixed to protected articles without impairing their appearance or use. Thirdly, the targets may be carried through the interrogation zone in any random orientation and along any path relative to the field generating and disturbance sensing antennas. Finally, the permissible power of the interrogating electromagnetic field is limited by governmental regulations.
The small amplitude disturbances produced by the targets used for electronic theft detection are especially difficult to sense and detect because of the fact that the detection system is usually required to operate in an environment in which a large amount of extraneous electromagnetic field energy, known as radio frequency noise, is also present. This noise includes natural or background noise (known as Gaussian noise), as well as so-called "man-made noise", such as that produced in the operation of electrical switches, fluorescent lighting, radio equipment and nearby electrical machinery. It has been found that even shopping carts produce radio frequency noise by virtue of the metal surfaces in the wheels rubbing against each other. The amplitude of this extraneous noise may be even greater than the amplitude of the signals produced by the targets themselves.
Various techniques have been proposed in the past for improviding the detectability of low signal level targets in a high noise level environment.
U.S. Pat. No. 3,696,379 proposes to use a second receiving antenna separate from the antenna which monitors the interrogation zone. When signals of a given amplitude are received by the second receiving antenna, a false alarm producing situation is considered to exist and the system is inhibited.
U.S. Pat. Nos. 3,624,631 and 3,810,147 propose to detect the spacing between signals produced when a target is interrogated by a swept frequency interrogating field.
Great Britain Pat. No. 1,292,380 proposes to open a gate in the receiver only during the intervals following transmission of interrogation signals.
U.S. Pat. Nos. 3,710,336; 3,781,860 and 3,868,669 and Great Britain Pat. Nos. 1,126,996 and 1,228,647 all propose to monitor a second frequency in addition to that produced by a true target and to inhibit the system if the other frequency signal level exceeds a predetermined threshold.
U.S. Pat. Nos. 2,794,974; 3,577,136; 3,218,556; 3,465,336 and 3,801,977 all propose to monitor a second or even a third frequency in addition to that produced by a true target and to inhibit the system except when the amplitude of the signal produced at the true target frequency is a predetermined amount above the amplitude of the other frequency signals.
In some of the foregoing patents more than one of the above described techniques are combined.
All of the foregoing prior art operates on the premise that a true target produces signals only at a given frequency, at a given location and at a given time, but that interfering noise signals, occurring at this same frequency, location and time are accompanied by other noise signals which occur at nearby frequencies, location or times. When signals at these other frequencies, locations or times are detected, they are used either to prevent, or to raise the threshold of, target detection. These prior techniques, however, fail to take into consideration that the target itself produces signals over a wide frequency spectrum; and, to the extent that the prior techniques ignore all but a small portion of the target frequency spectrum, or treat all but such small portion as noise signals, they are inherently limited as to how well they can discriminate a true target from extraneous noise.